1. Field of the Invention
The present invention relates to a multi-beam semiconductor layer having at least two juxtaposed light-emitting portions.
2. Description of the Related Art
Multi-beam semiconductor lasers having a plurality of juxtaposed light-emitting portions are used in various fields. For example, Japanese Unexamined Patent Application Publication No. 2002-324944 discloses a multi-beam semiconductor laser having a separated double heterojunction structure (hereinafter referred to as an “SDH multi-beam semiconductor laser”). The above patent application describes a problem generated when an SDH multi-beam semiconductor laser is mounted on a heat sink or the like by a junction-down method with solder or the like. More specifically, the following problem has been pointed out: When an SDH multi-beam semiconductor laser is mounted by a junction-down method, distortion may be generated in each of light-emitting portions (laminated structures of compound semiconductor layers), and stress may be generated in each of the light-emitting portions. As a result, a change in the refractive index due to a photoelastic effect is generated by the stress, and the state of polarization of a laser beam emitted from each of the light-emitting portions becomes different, that is, polarization of the laser beam is angularly rotated. Such an angular rotation of polarization causes a serious problem in that the transmittance or the reflectance becomes different among the light-emitting portions in a polarization optical system through which a laser beam passes.
This problem of angular rotation of polarization of a laser beam is a problem that does not readily occur in a junction-up-type SDH multi-beam semiconductor laser in the related art because such a semiconductor laser is free of stress. Here, the junction-up-type semiconductor laser is a semiconductor laser having a structure in which, in a light-emitting portion having a laminated structure including a first compound semiconductor layer of a first conductivity type, an active layer provided on the first compound semiconductor layer, and a second compound semiconductor layer of a second conductivity type different from the first conductivity type, the second compound semiconductor layer being provided on the active layer, the thickness of the first compound semiconductor layer is sufficiently larger than the thickness of the second compound semiconductor layer. In addition, in the SDH multi-beam semiconductor laser in the related art, the pitch of juxtaposed light-emitting portions is large; at least about 100 μm. Accordingly, the effect of the juxtaposed light-emitting portions on each other is small, and thus distortion is not readily generated in each of the light-emitting portions (laminated structures of compound semiconductor layers). Therefore, the problem of angular rotation of polarization of a laser beam does not readily occur (see “Reference Example” in the graph of FIG. 3). FIG. 16A shows a schematic plan view of an SDH multi-beam semiconductor laser in the related art having two light-emitting portions. FIG. 16B shows a schematic end view taken along line XVIB-XVIB in FIG. 16A. The two light-emitting portions are separated by a separation groove in order to electrically separate the light-emitting portions.